1. Field of the Invention
The present invention relates to a method for forming a Cu diffusion barrier metal for metal wiring structures that can be used favorably in the creation of fine semiconductor elements in general, as well as a structure of said Cu diffusion barrier metal.
2. Description of the Related Art
Ru film is drawing the attention for its ability to improve the adhesion property with respect to Cu and thereby enhance the wiring reliability significantly when a Ru film is formed at the boundary of Cu and barrier metal in a Cu wiring structure, which is a main wiring structure used in high-speed logic elements such as MPUs. Methods are studied whereby a Ru film comprising a Cu wiring liner is formed on top of a TaN film or WN film and then a Cu film is formed on top of the Ru film (one example of such method based on a combination of Ru and TaN is found in “Physical, Electrical, and Reliability Characterization of Ru for Cu Interconnects” by C-C Yong, et al., IITC 2006, pp. 187-189).
A Ru/TaN laminated film whose use as a Cu wiring liner is being examined tends to produce higher Cu wiring resistivity because the thicker the film, the smaller the Cu wiring volume becomes. Accordingly, reducing the film thickness of the Cu wiring liner is essential in reducing the resistivity resulting from smaller wiring widths and use of thinner Cu wires. Accordingly, forming a film offering greater coverage using the atomic layer deposition method is drawing more attention compared with the traditional PVD method.
As far as adhesion property is concerned, traditionally a Cu liner such as a PVD-Ta film or PVD-TaN film can be formed in a manner similar to driving a wedge, because physically accelerated ions collide with the surface to be coated in the PVD method. Accordingly, good adhesion property can be maintained, even when a PVD-TaN film is formed on a Cu film in the bottom wiring layer, because the TaN film is formed in a manner biting into the Cu film. On the other hand, a TaN film formed by the atomic layer deposition method has been shown to reduce the adhesion property with respect to Cu wiring. Unlike in PVD, under the atomic layer deposition method where a film is formed by means of chemical reaction an area where a mixture of different atoms is present is not formed between the bottom Cu wiring layer and the metal film comprising the Cu liner, and therefore if a TaN film, TaNC film or other Cu barrier metal is formed, good adhesion property is difficult to maintain compared to when the PVD method is used. Sandwiching a Ru film between the Cu film and the TaN film or TaNC film is expected to produce good adhesion property.
If a multi-layer Cu wiring structure is formed, many via holes must be formed that serve as connection holes through which to connect the top and bottom Cu wirings. In general, a dual damascene structure is used in these applications, and thus the following explanation assumes use of a dual damascene structure. A Cu film is formed over the via holes and trenches in the bottom Cu wiring layer, after which Cu wiring pattern is formed. This is to prevent Cu from diffusing into the inter-layer insulation film and consequently increasing the leak current to cause insulation failure. If this Cu barrier metal has poor adhesion property with respect to Cu, however, the Cu film will separate during the reliability test and voids will form. Accordingly, it is desirable to form a Ru film at the interface of Cu wiring and Cu barrier metal, as shown in known examples. However, forming a Ru film over the Cu wiring exposed at the bottom of via holes normally results in poor adhesion property at the interface of inter-layer insulation film and Ru. As a method to solve this problem, Seong-Jun Jeong et al. made a presentation under the following title at ALD Conference 2006: “Plasma Enhanced Atomic Layer Deposition of Ru—TaN Thin Films for the Application of Cu Diffusion Barrier.”
The above paper proposes a method to create a metal alloy film comprised of Ta and Ru between the Cu film of the bottom layer and the Cu film of the top layer by repeating a step to introduce Ta material, step to purge Ta material, step to introduce hydrogen gas and apply high-frequency plasma, step to purge hydrogen gas, step to introduce Ru material, step to purge Ru material, step to introduce ammonia gas and apply high-frequency plasma, and step to purge ammonia gas. In this case, the researches indicate that good adhesion property can be achieved with respect to both the top and bottom Cu films as well as dielectric layers because an alloy of Ta and Ru is formed. Based on the result of an experiment conducted by the inventors of the present invention, however, it is difficult to break down the material and form a Ru film, unless NH3 plasma is used, when Ru(EtCp)2 is to be used as an organic metal material containing Ru metal atoms as indicated in the aforementioned paper. However, using NH3 plasma accelerates the nitriding of the Ta layer formed at the bottom and causes the film resistivity to increase significantly. In other words, Ta will be nitrided by ammonia plasma to form a high-resistivity component of Ta3N5 or a high-resistivity film of relatively high nitrogen content. Accordingly, applying the present invention to via holes connecting the Cu wirings of the top and bottom layers, which is the objective of the aforementioned known example, increases the via resistivity and reduces the wiring reliability.
To lower the wiring resistivity, the amount of Ru must be increased. Accordingly, adhesion property with respect to the insulation film tends to decrease when the amount of Ru is increased. Particularly when Ru is used as a Cu wiring liner, the interface with the bottom layer consists of Cu and an inter-layer insulation film, while the interface with the top layer consists of a Cu wiring only. This difference makes it difficult to achieve good adhesion property at the top and bottom interfaces using a film of the same quality.
In Korean Patent Publication No. 10-2005-0103373, a method is presented whereby a film containing amorphous Ru and Ta is formed and used as a Cu barrier film by repeating the first atomic layer deposition process consisting of a step to introduce Ru material, step to purge Ru material, step to introduce ammonia gas and apply high-frequency plasma, and step to purge ammonia gas, as well as the second atomic layer deposition process consisting of a step to introduce Ta material, step to purge Ta material, step to introduce hydrogen gas and apply high-frequency plasma, and step to purge hydrogen gas.
As is the case with the first known example, however, under this method it is also difficult to achieve good wiring reliability for the top and bottom layers with an alloy of amorphous Ta and Ru alone, because Ru offers better adhesion property with respect to Cu in the top layer, while TaN offers better adhesion property with respect to the inter-layer insulation film in the bottom layer. Even if an Ta—Ru alloy film is effective as a copper diffusion barrier, problems occur easily when current is applied, such as the barrier film separating from the Cu film to form voids or the barrier film and inter-layer insulation film separating from each other. Accordingly, a desirable structure is one that maintains adhesion property with respect to the bottom Cu wiring layer, offers excellent Cu diffusion barrier property, and also provides good adhesion property with respect to the top Cu wiring layer.